The present invention relates to a process for polymerizing cyclic ethers over a heterogeneous catalyst comprising one or more pillared interlayered clays (PILCs).
Polytetrahydrofuran (PTHF), also known as poly(oxybutylene glycol), is an intermediate for the preparation of polyurethane, polyester and polyamide elastomers, where it is used as diol component. The incorporation of PTHF renders these polymers soft and flexible, which is why PTHF is also known as a soft segment component for these polymers. Polytetrahydrofuran monoesters of monocarboxylic acids are used, for example, as plasticizers (U.S. Pat. No. 4,482,411), impregnating agents, monomers (EP-A 286 454), emulsifiers and dispersants, and are also employed for deinking in the recycling of waste paper.
The cationic polymerization of tetrahydrofuran (THF) using catalysts has been described by Meerwein et al. (Meerwein et al. (1960) Angew. Chem. 72, 927). The catalysts used either are preshaped catalysts or are formed in situ in the reaction mixture. In the latter case, oxonium ions which initiate the THF polymerization are generated in the reaction mixture using strong Lewis acids such as boron trichloride, aluminum trichloride, tin tetrachloride, antimony pentachloride, ferric chloride or phosphorus pentafluoride or strong Bronsted acids such as perchloric acid, tetrafluoroboric acid, fluorosulfonic acid, chlorosulfonic acid, hexachlorostannic acid, iodic acid, hexachloroantimonic acid or tetrachloroferric acid, and using reactive compounds called promoters such as alkylene oxides, eg. ethylene oxide, propylene oxide, epichlorohydrin or butylene oxide, oxetanes, orthoesters, acetals, xcex1-halo ethers, benzyl halides, triarylmethyl halides, acid chlorides, xcex2-lactones, carboxylic anhydrides, thionyl chloride, phosphorus oxychloride or sulfonic acid halides. However, only a few of the multiplicity of catalyst systems have gained industrial importance since some of them are highly corrosive and/or in the course of PTHF preparation give rise to colored products of limited utility. Moreover, many of these catalyst systems are not true catalysts but must be employed in stoichiometric amounts relative to the macromolecule to be prepared and are consumed in the course of the polymerization. The preparation of PTHF using fluorosulfonic acid as catalyst according to U.S. Pat. No. 3,358,042, for instance, requires the use of about two molecules of fluorosulfonic acid for each molecule of PTHF. The use of halogen-containing catalysts has the particular disadvantage that halogenated byproducts are formed in PTHF polymerization which are difficult to remove from pure PTHF and adversely affect the properties thereof.
In the preparation of PTHF in the presence of the abovementioned promoters, these promoters are incorporated into the PTHF molecule as telogens so that the primary product of THF polymerization is not PTHF but a PTHF derivative, for example a PTHF diester or disulfonate from which PTHF has to be liberated in a further reaction, for example by saponification or transesterification (cf. U.S. Pat. No. 2,499,725 and DEA 2 760 272). Telogens are generally compounds which cause chain termination and/or chain transfer in the polymerization. If alkylene oxides are used as promoters, these also act as comonomers and are incorporated into the polymer which leads to the formation of THF-alkylene oxide copolymers which have different application properties than PTHF.
PTHF may be prepared in one step by polymerizing THF in the presence of water, 1,4-butanediol or low molecular weight PTHF oligomers. If 2-butyne-1,4-diol is used as telogen, copolymers of THF and 2-butyne-1,4-diol are produced which, however, may be converted into PTHF by hydrogenating the triple bonds contained therein.
U.S. Pat. No. 5,149,862 discloses the use of sulfate doped zirconium dioxide as acidic heterogeneous polymerization catalyst which is insoluble in the reaction medium. A mixture of acetic acid and acetic anhydride is added to the reaction medium to accelerate the reaction, since the polymerization is very slow without these promoters and conversion over 19 hours is only 6%. This process gives rise to PTHF diacetates which have to be converted into PTHF subsequently by saponification or transesterification.
PTHF diesters are likewise formed in the polymerization of THF using bleaching earth catalysts, as described in EP-A 0 003 112.
U.S. Pat. No. 4,303,782 uses zeolites for the preparation of PTHF. The THF polymers obtainable by this process have very high average molecular weights (Mn=250.000-500.000 D) and have not found general acceptance for the above-mentioned applications. The process has therefore likewise attained no industrial importance.
DE 4 433 606 describes for example the preparation of PTHF in one step by polymerizing THF over heterogeneous supported catalysts which comprise a catalytically active amount of an oxygen-containing molybdenum and/or tungsten compound on an oxidic support material and which have been calcined at from 500 to 1000xc2x0 C. after application of the precursor compounds of the oxygen-containing molybdenum and/or tungsten compounds onto the support material precursor. These catalysts have the disadvantage that expensive zirconium dioxide is used as support material.
It is an object of the present invention to provide a process which enables the polymerization of cyclic ethers to be performed in an advantageous manner, especially with high space-time yields, and without the disadvantages described above.
We have found that this object is achieved by a process for polymerizing cyclic ethers over a heterogeneous catalyst comprising one or more pillared interlayered clays (PILCs), which are known from Figueras, F., Catal. Rev. Sci Eng. 30(3) (1988), 457 or Jones, Catal. Today 2 (1988) 357, for example.
PILCs are generally layer structures intercalated with one or more metal compounds in the form of pillars (cf., for example, FIG. 2 in Figueras, F. (1988), supra). The interlayer distance is generally from about 4 to 80 xc3x85, preferably from about 8 to 30 xc3x85, especially from about 8-25 xc3x85. The space which is opened up between the layer structures by the intercalated metal compounds is available as pore volume for the reactants of the polymerization of the invention. Additional pore volume is created, for example, by delamination, ie. xe2x80x9chouse of cardsxe2x80x9d structures are formed.
Preferred metal compounds for the pillars include oxides and/or sulfides of elements of main groups III and IV of the Periodic Table of the Elements, in particular of aluminum, gallium, indium, thallium, silicon, germanium, tin or lead, especially aluminum, gallium or silicon, or of elements of the transition groups, preferably of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese or iron, especially of titanium, zircomium, vanadium, tantalum, chromium or iron, which may be present as mixtures with one another or as mixtures with other oxides and/or sulfides, eg. of magnesium, boron, cobalt or nickel. Oxidic pillars are preferred.
Examples of useful metal oxides include Al2O3, ZrO2, TiO2, Cr2O3, Ga2O3, SiO2, Ta2O5, Fe2O3, and V2O5. Examples of other oxides that may be present include MgO, B2O3, Co2O3 or NiO. A mixture of Al2O3 and MgO which results in a mixed aluminum/magnesium oxide is especially preferred. Examples of sulfides include Fe2S3.
Metal compounds having perovskite structure, for example LaCoO3, LaNiO3, LaMnO3 and/or LaCuO3, are also suitable as pillars (cf. WO 92/00808, for example).
The amount of intercalated metal is preferably about 1-50% by weight, in particular 2-35% by weight, based on the finished PILC and calculated as % by weight of metal.
Layer compounds suitable for preparing PILCs are preferably sheet silicates, especially clays. Examples of clay minerals include smectite minerals such as montmorillonite in pure form or as bentonite constituent. Other examples of smectites are beidellite, hectorite, nontronite, sauconite or saponite (cf., for example, U.S. Pat. No. 5,409,597, Table 1). Other examples of clay minerals are vermiculite, mica and taeniolite and of sheet silicates are kanemite, ilerite, magadiite, makatite or kenyaite.
Other examples of suitable layer compounds are xcex1-zirconium phosphate, tetrasilicon mica, brucite, silicic acid type I or rectorite (cf. Vaughan, D. E. W. (1988) xe2x80x9cDevelopments in Pillared Interlayered Claysxe2x80x9d in Perspectives in Molecular Sieve Science (Flank, W. H. and Whyte, Th. E. Jr. eds.), for example) ACS Symposium Series, 368, 308-323, Chapter 19, American Chemical Society; Szostak, R. and Ingram, C. (1995) xe2x80x9cPillared Layered Structures (PLS): From Microporous to Nano-phase Materialsxe2x80x9d in Catalysis by Microporous Materials, Studies in Surface Science and Catalysis vol. 94, 13 (Beyer, H. K. et al., eds.) Elsevier Science B. V.).
For simplicity, PILC as used herein also includes other pillared layered structures not prepared from clay minerals.
PILCs are generally prepared from commercially available naturally occuring or synthetic, untreated or pretreated layer compounds (cf. Mokaya, R. and Jones, W. (1994) J. Chem. Soc. Chem. Commun., 929-930 or WO 95/14530, for example).
One or more metal compounds may be intercalated into pretreated or untreated layer compounds, for example, by the following generally known process (cf. U.S. Pat. No. 4,238,364 or WO 95/14530, for example).
Firstly, the layer compound(s) which is/are generally negatively charged is/are dispersed in a dispersion medium such as water and subsequently mixed with a solution comprising one or more oligomeric hydroxide ions of the metals cited above, the ions being generally positively charged. The metal hydroxide solution may, for example, be prepared by alkaline hydrolysis of a corresponding salt solution by methods known to those skilled in the art. Useful starting compounds include AlCl3, aluminum chlorohydrate, aluminum nitrate, aluminum acetate, zirconyl chloride, zirconyl nitrate, titanyl chloride, titanyl nitrate, titanium tetrachloride, chromium(III) nitrate, iron(III) nitrate, tin(IV) chloride, tin(IV) nitrate, tin(IV) acetate. Solutions of these salts are used to prepare the corresponding hydroxides, for example using aqueous ammonia solution, sodium hydroxide solution or sodium carbonate solution. Alternatively, the hydroxides may be obtained by adding diluted or weak acids such as acetic acid to water soluble hydroxo complexes of the corresponding metals. It is likewise possible to obtain the hydroxides by hydrolysis of organometallic compounds, for example of the alcoholates of the corresponding metals, eg. zirconium tetraethanolate, zirconium tetraisopropylate, titanium tetramethanolate, titanium tetraisopropylate. For the purposes of the present invention, xe2x80x9chydroxidesxe2x80x9d is a collective term for the oligomeric ions of the metals cited above which may, for example, also contain oxide hydrates, polymeric hydroxo complexes or other anions such as chloride or alcoholate ions. The suspensions are then stirred at, for example, about 0-100xc2x0 C., preferably at about 20-95xc2x0 C., for about 30 min-100 h and the layer compound is removed by, for example, filtration or centrifugation, washed eg. with deionized water and usually dried in air or under an inert gas atmosphere, eg. nitrogen, at about 100-160xc2x0 C. and calcined at about 150-600xc2x0 C., preferably about 200-500xc2x0 C., for about 2-16 hours. Freeze-drying is also possible. Examples of positively charged metal hydroxides are [Al13O4(OH)24(H2O)12]7+ or [Zr4(OH)8(H2O) 16]8+ leading to aluminum oxide or zirconium oxide compounds (xe2x80x9cpillarsxe2x80x9d), ie. Al- or Zr-PILCs, following intercalation and calcination.
In a further embodiment, the layer compounds are treated with one or more acids prior to or after intercalation of one or more metal compounds and prior to the shaping procedure which is described in detail below, since the acid treatment may increase the pore volume and the activity of the PILCs. Preference is given to acid treatment using an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, phosphoric acid and/or an organic acid such as oxalic acid. Acid treatment generally utilizes about 0.001-20 N, preferably about 0.1-10 N, acid at about 0-150xc2x0 C. for about 1-100 hours, preferably about 1-24 hours, in an aqueous slurry of the layer mineral. Following removal and washing, the material is generally calcined at about 150-600xc2x0 C., preferably at about 200-500xc2x0 C., for about 2-16 hours.
In an alternative embodiment, the layer compounds may additionally be acid treated after the shaping procedure which is described in detail below in order to exchange residual alkali metal and alkaline earth metal ions for hydrogen ions. In this case, the layer compound is generally acid treated using an acid of about 3-25% strength at about 60-80xc2x0 C. for about 1-3 hours, dried at about 100-160xc2x0 C. and calcined at about 200-600 C. In particular, the acid treatment of ZrO2-, TiO2- or Fe2O3-PILCs, for example with sulfuric acid, gives rise to PILCs having sulfated metal oxide pillars and particular thermal stability (Farfan-Torres, E. M. and Grange, P., Catal. Sci. Technol. 1 (1991) 103-109).
A further way of exchanging residual alkali metal and alkaline earth metal ions for hydrogen ions is treatment with ammonium and/or amine salts. To this end, the layer compounds are treated, prior to or after intercalation with one or more metal compounds, with an about 0.1-40% by weight strength, preferably about 5-30% by weight strength, ammonium salt solution such as an ammonium chloride solution and/or an ammonium nitrate solution and/or a salt solution of a volatile amine, for example ethylamine, in an aqueous slurry of the layer compound at about 0-100xc2x0 C. for about 1-100 hours, preferably about 1-24 hours. Following removal and washing, the material is generally calcined at about 150-600xc2x0 C., preferably at about 200-500xc2x0 C., for about 2-16 hours to remove again the ammonia or the volatile amine, respectively.
In another embodiment, the layer mineral is fluorinated, prior to or after intercalation, using one or more fluorides, for example ammonium fluoride, either replacing the hydroxyl groups of the layer mineral by fluoride (cf. U.S. Pat. No. 5,308,812, for example) and/or fluorinating the intercalated metal oxides (cf. U.S. Pat. No. 5 409 597, for example).
In another embodiment, the layer compound was additionally doped with metal ions, in particular transition metal ions, for example with titanium, zirconium, niobium, molybdenum, tungsten, rhenium, nickel, iron, cobalt ions, and/or with rare earth metal ions such as cerium, yttrium and/or lanthanum ions, prior to or after intercalation of one or more metal oxides and prior to or after the shaping procedure (cf. U.S. Pat. No. 4,238,364 or Jiang et al. in Proc. 9th Int. Zeolite Conf. 2 (1992) 631-638, for example). In a preferred embodiment, the preshaped PILC is placed in a flow tube and a solution of metal ions, for example in the form of a halide, an acetate, an oxalate, a citrate and/or a nitrate, is passed over it at about 20-100xc2x0 C. Another way of doping the catalysts is impregnation of the PILC with a solution, for example an aqueous or alcoholic solution, of the transition metal salts described above. Subsequently, the material is dried and additionally calcined under the conditions which have already been described in detail above, if desired. It may also be advantageous to subject the metal-doped PILC to an aftertreatment with hydrogen and/or steam.
The PILC to be used for the process of the invention can generally either be shaped as such, or with a binder preferably in a ratio of from about 98:2 to about 40:60, to give shaped articles, for example extrudates or pellets. Suitable binders are various aluminas, preferably boehmite (AlOOH), amorphous aluminosilicates, silica, preferably finely divided silica, finely divided titania and/or clays such as kaolin. After the shaping procedure, the extrudates or compacts are advantageously dried at about 110-120xc2x0 C. overnight and then calcined at about 150-600xc2x0 C., preferably at about 200-500xc2x0 C. for about 2-16 hours, it being possible for the calcination to take place directly in the polymerization reactor. If the process of the invention is carried out in suspension mode, the heterogeneous catalysts may be used as a powder or, if the heterogeneous catalyst is arranged in a fixed bed, as shaped articles, for example in the form of cylinders, spheres or granules. The arrangement of the heterogeneous catalyst in a fixed bed is preferred, in particular if, for example, loop reactors are used or if the process is carried out continuously.
The heterogeneous catalysts described above generally have a BET surface area of about 50-400 m2gxe2x88x921, preferably of about 60-300 m2gxe2x88x921, in particular of 100-300 m2gxe2x88x921, and are surprisingly very advantageous for the polymerization of cyclic ethers. This property was particularly surprising since the catalysts have hitherto been used mostly in petrochemical processes only, for example as catalysts for alkylations, isomerizations or cracking of hydrocarbons, ie. in processes which are not related to the process of the invention.
Useful cyclic ethers are in particular cyclic ethers of the formula (I) 
where R1 is a bond or 1-8 carbon atoms, preferably 1-4 carbon atoms, especially 2 carbon atoms, which may be substituted with a radical R6 and/or R7, and R2, R3, R4, R5, R6 and R7 are each independently of one another hydrogen, a saturated or mono- or polyunsaturated alkyl group having 1-4 carbon atoms or an aryl group having 6 carbon atoms, where R2, R3, R4, R5, R6 and/or R7 may be linked via 2-8 carbon atoms, preferably 4-5 carbon atoms, which may be substituted with one or more radicals like R6 and/or R7. Particularly preferred cyclic ethers include ethylene oxide, propylene oxide, oxetane, tetrahydrofuran (THF), tetrahydropyran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran and styrene oxide, especially THF, 2-methyltetrahydrofuran or 3-methyltetrahydrofuran, or mixtures of one or more of the cyclic ethers cited with at least one telogen compound selected from the group consisting of water, alkanediols, alkenediols or alkynediols having each 1-12 carbon atoms, preferably 1-6 carbon atoms, in particular 1-4 carbon atoms, especially water, 1,4-butanediol and/or 2-butyne-1,4-diol, polytetrahydrofuran (PTHF) having a molecular weight of about 200-700 dalton, a monocarboxylic acid having 1-10 carbon atoms, preferably 1-8 carbon atoms, particularly formic acid, acetic acid, propionic acid, 2-ethylhexanoic acid, acrylic acid and/or methacrylic acid, and/or a carboxylic anhydride of monocarboxylic acids having 2-20 carbon atoms, preferably 2-8 carbon atoms, in particular acetic anhydride, propionic anhydride and/or butyric anhydride, especially water, 1,4-butanediol, formic acid, acetic acid, 2-butyne-1,4-diol and/or acetic anhydride. Particular preference is given to a mixture of THF and 1,4-butanediol, preferably of 1 mol THF and about 0.1-15 mol 1,4-butanediol, of THF and a 1,4-butanediol/water mixture, of THF and low molecular weight PTHF or of THF and acetic anhydride.
In principle it is possible to use any cyclic ether for the catalytic polymerization, ie. including commercially available cyclic ethers or cyclic ethers which have been prepurified by acid treatment or distillation. THF prepurified by acid treatment is described in EP-A-0 003 112, for example.
The telogens are preferably dissolved in the cyclic ether itself, for example in THF. Furthermore, it is possible to control the average molecular weight of the polymerization product via the amount of telogen used. The higher the telogen content of the reaction mixture, the lower the average molecular weight of the polymerization product. It is thereby possible, for example, to prepare PTHF or the corresponding PTHF copolymers having average molecular weights of about 250-10000 in a controlled manner depending on the telogen content of the polymerization mixture. The process of the invention is preferably employed to prepare PTHF or the corresponding PTHF copolymers or the corresponding derivatives having average molecular weights of about 500-10000 dalton, especially about 650-5000 dalton. To this end, the corresponding telogen is added in an amount of about 0.01-20 mol %, preferably about 0.05-10 mol % and more preferably about 0.1-8 mol %, based on the amount of cyclic ether, eg. THF, used.
For instance, the telogen 2-butyne-1,4-diol is used for the catalytic preparation of polyoxyalkylene glycols containing Cxe2x80x94C triple bonds or Cxe2x80x94C double bonds as described in detail in WO 96/27626, for example, or for the catalytic preparation of a copolymer of THF and 2-butyne-1,4-diol as described in detail in DE 195 275 32, for example. Otherwise, reference is also made to DE 44 33 606 or WO 96/09335 where the catalytic preparation of PTHF and PTHF copolymers is described in detail.
The catalytic polymerization is generally carried out at about 0-80xc2x0 C., preferably at from about 25xc2x0 C. to the boiling temperature of the reaction mixture, for THF up to 66xc2x0 C., for example. The pressure applied is generally not critical for a successful polymerization by the process of the invention, and the polymerization is generally carried out at atmospheric pressure or under autogenous pressure of the polymerization system. To prevent the formation of ether peroxides, preference is generally given to polymerizing under an inert gas atmosphere, eg. nitrogen, hydrogen, carbon dioxide or noble gases such as argon, preferably nitrogen.
The process of the invention can be carried out continously or batchwise, a continuous process being generally preferred for economic reasons. In the batch process, the cyclic ether(s), eg. THF, the corresponding telogen(s) and the catalyst(s) are generally reacted at the abovementioned temperatures in a stirred tank or in a loop reactor until the desired conversion of cyclic ether is achieved. The reaction time may be about 0.5-40 hours, preferably about 1-30 hours, depending on the amount of catalyst added. The catalysts are generally used in an amount of about 1-90% by weight, preferably about 4-70% by weight, in particular about 8-60% by weight, based on the weight of the cyclic ether(s), eg. THF.
The reaction effluent is worked up, for example in the batchwise process, by removing the catalyst present in the effluent, conveniently by filtration, decanting or centrifugation, and generally distilled, and unconverted THF is usually distilled off and low molecular weight PTHF may be removed from the polymer by distillation under reduced pressure, if desired. The low molecular weight PTHF may be recycled into the polymerization to act as telogen and converted into PTHF having a higher molecular weight.
Products of the catalytic polymerization reaction are PTHF, PTHF derivatives and/or copolymers of THF and at least one of the abovementioned compounds, for example a PTHF monoester derived from the reaction of THF and a monocarboxylic acid, a PTHF diester derived from the reaction of THF and a carboxylic anhydride, or THF/butynediol copolymers derived from the reaction of THF and 2-butyne-1,4-diol. The derivatives or copolymers may then be converted directly into PTHF by saponification or hydrogenation by generally known methods which have already been mentioned above.
It is particularly surprising that the polymerization of cyclic ethers, in particular the polymerization of THF, especially using water and/or 1,4-butanediol and/or low molecular weight PTHF and/or acetic anhydride as telogen, can be achieved according to the process of the invention with high space-time yields in one step and therefore in a particularly advantageous manner. The process of the invention is also particularly advantageous in that it utilizes low molecular weight, open-chain PTHF having a molecular weight of about 200-700 dalton (low molecular weight PTHF) as telogen. Since PTHF and 1,4-butanediol have two hydroxyl groups, they are not only incorporated at the ends of the PTHF chain as telogens, but also incorporated into the PTHF chain as monomers.